In many situations, a passageway extending between two different environments must be closed off to prevent materials present in one environment from passing over to the other environment. At the same time, in many situations the passageway must be periodically opened up some or all of the way so as to permit materials to be passed from one environment to the other.
Furthermore, in some situations tools must be passed through the passageway--and hence through any seals already established across the passageway--so that their distal working ends are positioned in one environment while their proximal actuating ends are positioned in the other environment. Such a deployment is frequently very important, since it permits a person to perform tasks in one environment using the distal working end of the tool while manipulating the proximal actuating end of the tool in the other environment.
Unfortunately, however, it can be quite difficult to establish a secure seal about the periphery of a tool as it is inserted across the passageway and to thereafter reliably maintain that seal during the tool's use, particularly inasmuch as the tool may be moved about axially and/or radially during use, and particularly inasmuch as the tool may have a different cross-sectional profile along its length. Furthermore, in some circumstances it may be necessary to replace one deployed tool with another tool. In this case it can be quite difficult to reestablish a secure seal across the passageway when the initial tool is withdrawn, and it can be quite difficult to establish a secure seal with the replacement tool as it is emplaced. It should be appreciated that this can be particularly true in the case where the replacement tool has a cross-sectional profile which differs from that of the initial tool.